This invention relates generally to integrated optics, and, more particularly, to optical devices for coupling a single input waveguide to a number of output waveguides with a desired apportionment of total power. Coupling devices performing this function are usually referred to as star couplers or 1.times.N star couplers, to signify that one input channel is coupled to N output channels, in predetermined, most commonly equal, power apportionment. This type of coupling device is often needed in certain types of optical communications networks.
In the past, star couplers of the single-mode type have been fabricated in non-integrated form by encapsulated etching of a bundle of a single-mode fibers. However, couplers cannot be reliably made by this technique in a reproducible fashion. Also, such couplers do not reliably preserve the polarization properties of light transmitted through them. Some star couplers have been fabricated using techniques of "integrated optics." This term encompasses the integration of optical devices on small substrates, using such methods as photolithography, in much the same manner that they are employed in the fabrication of integrated circuitry.
Limited forms of star couplers have been made using parallel couplers, in which light from a first waveguide is coupled to a second waveguide through closely spaced parallel portions of the two waveguides, or using Y-junction branching circuits to bifurcate the transmitted light into two approximately equal portions. U.S. Pat. No. 4,165,225 to Auracher et al. discloses a coupler formed from a number of Y junctions connected in a tree-like structure to divide one input signal into a plurality of output signals of approximately equal power. Unfortunately, there is an inherent difficulty in making consistently equal power splits with Y junctions. Also, arrangements like the Auracher one necessitate 2N-2 bends in a 1.times.N coupler, and are therefore subject to considerable losses.
Another coupler of the prior art, as taught by U.S. Pat. No. 4,262,995 to Tangonan, has a plurality of waveguides that open into a single mirrored "mixing chamber." Light emerging into the chamber from one of the waveguides is mixed by repeated reflections and launched into the other waveguides in practically equal portions. However, the device is unsuitable for single-mode operation, and is subject to relatively high losses.
Others in the field have proposed using arcuate waveguides, in integrated optics form, as coupling devices. U.S. Pat. No. 4,116,530 to Bellavance et al. teaches a technique for fabricating such arcuate waveguides. U.S. Pat. No. 3,977,762 to Sandbank also discloses a device using arcuate waveguides, but not in the context of star couplers.
It will be appreciated from the foregoing that there is still a significant need in the field of integrated optics for a star coupler compatible with single-mode fibers and with minimal insertion losses. Ideally, the star coupler should be easily and repeatedly fabricated to produce a desired division or equality of power outputs, and should have strong polarization preservation properties. The present invention is directed to these ends.